1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus and an image forming program therefor, for performing a halftone process using halftone spots each consisting of multiple dot images. In particular, the present invention pertains to an innovative image forming apparatus and an image forming program product therefor, used to improve image quality by forming virtual dots at a desired printed dot gain at desired locations in a pixel area.
The present application is based on Japanese Patent Application No. 2000-235214, which is incorporated herein by reference.
2. Description of the Related Art
To reproduce a color image, a color electrophotographic apparatus that is widely employed in color printers and color copiers uses cyan (C), magenta (M), yellow (Y) and black (K) toner to develop a latent image formed by exposing a photosensitive member, and then, to prepare the final image, transfers the toner image to an image bearing member, such as a paper sheet. A laser beam printer that employs a laser beam to form a latent image on a photosensitive member controls laser beam emissions for individually scanning pixels arranged in the main scanning direction, in which the laser beam is emitted, and in the sub-scanning direction, in which the image bearing member is fed. Above all, a laser beam printer of a type that can change the width of a pulse used for emitting a laser beam can variously change the region in a pixel area that is irradiated by the laser beam. Even when only a small number of pixels are present in a unit area, this type of laser beam printer can provide a color image reproduction having a higher resolution and a larger number of gray levels.
For a laser beam printer of the pulse width change type, as one of the density tone reproduction methods used for images, a multi-level dithering, half-tone spot half-toning printing method is employed. According to the multi-level dithering method, upon the receipt of signals containing gray level data for colors, a transformation table, called a look-up table, in which reproduction data are stored is referred to, and appropriate data are extracted to determine the sizes of virtual dots and their locations in individual pixel areas. Since multiple levels, extending from 0 to a maximum value, are set as sizes, the outputs of the pixels are “multi-leveled”.
A “virtual dot” is defined as an area that is scanned by a laser beam in the process of forming a final, toner “dot image”. The size of such a virtual dot in the main scanning direction is equal to the product of the time and the speed whereat the laser beam is driven, and in the sub-scanning direction, is equal to the length of a pixel area. Since for the above and following reasons the shapes of virtual dots making up a final “dot image” differ, these dots are described as being individually distinguished. In the processing for producing a dot image, a laser beam is driven that irradiates virtual dots in pixel areas on a photosensitive member, and forms an irradiated region that, depending on the size and the leading and trailing characteristics of the laser beam, is shaped like a smeared virtual dot. In the thus prepared irradiated region on the photosensitive member, a latent image is produced that can be developed using toner and transferred to an image bearing member, such as paper, to produce a final, dot image. The shape of this dot image reflects the shapes of the virtual dots, for which further changes are engendered by the scattering of toner. Thus, since as is described above a dot image is composed of virtual dots that have undergone changes, and since the virtual dot changes are induced by the electrophotographic processing that is performed, the shape of a dot image can be controlled by processing adjustments made to alter the shapes of virtual dots.
According to the halftone-spot half toning method, a halftone spot that is formed consists of a dot image in a single pixel, or a group of dot images distributed among multiple adjacent pixels, and the gray level of an image is reproduced in accordance with the size of halftone spots. That is, as the density value of the gray level data for each pixel is increased, a virtual dot is generated, as is a growth core for a halftone spot in a final image. Then, when the density value of the gray level data is further increased, the number and the ratio of the virtual dots is likewise increased, while the size of the halftone spot is gradually enlarged. Therefore, in accordance with the method for growing a halftone spot in consonance with an increase in the density value of the gray level data that is input, the area of a virtual dot grows rapidly at pixels near the center of the halftone spot (near the growth core), and grows slowly at pixels around the circumference of the halftone spot (distant from the growth core).
In order to form a high quality image using an electrophotographic apparatus, the method used to grow a dot image in a pixel area must be optimized. For example, for most electrophotographic apparatuses it is well known that a high quality image can be obtained by using a method that increases the area of an isolated halftone spot at a low gray level, and by increasing the area size by connecting adjacent halftone spots at a high gray level and forming a line along a screen angle. One of the reasons for this is as follows. As previously described, since an electrophotographic apparatus employs light, such as a laser beam, to form a latent image, the portion of a latent image around a halftone spot is blurred, and since the latent image is developed by attaching charged toner that is adversely affected by the ambient temperature and humidity, image reproductivity for the image developed around the halftone spot is deteriorated. Therefore, when a line is formed by connecting halftone spots to the extent as possible, the distance around the circumference of a halftone spot can be reduced, and the image reproductivity by the development can be improved. Thus, at a low gray level whereat the halftone spots can not be connected, the area of each separate halftone spot is increased, and at an intermediate gray level or higher, a line is formed by connecting the halftone spots arranged along the screen angle, and the area of the line is increased.
When such a halftone-spot growing method is employed, for the virtual dot formed in the pixel area it is necessary not only that its size be changed but also that the position (development position) thereof be changed in accordance with the gray level.
However, the conventional image forming method takes into consideration only the changing of a laser driving pulse corresponding to the size of the virtual dot in the pixel area, and does not take into consideration the changing in the position of the virtual dot in the pixel area. Therefore, the changing of the position of the virtual dot in the pixel area in accordance with the input gray level, as described above, is not performed.